This is a hotly contested topic here where a lot of people have had their opinions influenced (positively or negatively) by stupid stores selling gimmicks that they call ram air and the "The Fast and the Furious". If you really want to understand how a good ram air system can work on a car, take a look at how the inlet tracts on motorcycles are designed. Here are some things to note....
1) Huge inlet in the front (not somewhere in the engine! That's not a ram, short ram, or cold air intake. That's crap!)
2) A HUGE airbox
3) Sealed
You have to remember that these things don't generate significant boost (Don't know if you knew that or not.
). But they
can (if well designed) provide a slight over pressure effect. Slight, but not much at all. If you've ever driven your car around and noticed how well it ran in the region of 0 in / 0 psi, you can get an idea of how atmospheric motors benefit from this.
What's still hotly contested around here is whether or not this is worth it on a turbo car. I say yes. The reason being that when your turbo really starts to spin, it's going to need huge amounts of air at the inlet to generate that 20psi you and I both know you want.
If the inlet tract upstream of the turbine is poorly designed, how much harder does the compressor have to work to generate that 20psi? Especially considering that in a non-ram air setup, the inlet tract essentially becomes a vacum cleaner tube right?
A well designed inlet tract will help feed that beast.
As for how big you should make the pipe, I don't know. I'm using two 3" pipes to feed an airbox upstream of my MAF.
And if you are worried about water, just don't have the inlet so low. Some people cut holes through their bumper. Hint.... hint.....
If you still at risk or getting water in then, you've obviously ignored warnings and common sense and ventured out into a storm that's laying serious waste to your surroundings.
Cheers